Heart block

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]


Heart block is defined as impaired or abnormal conduction of electrical impulses in the heart. With each heartbeat, an electrical signal spreads across the heart from the upper (atria) to the lower chambers (ventricles). As it travels, the signal causes the heart to contract and pump blood. This process repeats with each new heartbeat. Heart block occurs if the electrical signal is slowed or disrupted as it moves from the upper to the lower chambers of the heart. This abnormality can occur in the heart muscle (myocardium) or in the specialized electrical conduction system of the heart.


A heart block can be a blockage at any level of the electrical conduction system of the heart:

Clinically speaking, most of the important heart blocks are AV nodal blocks and Infra-Hisian blocks.

Types of SA nodal blocks

The SA nodal blocks rarely give symptoms. This is because if an individual had complete block at this level of the conduction system (which is uncommon), the secondary pacemaker of the heart would be at the AV node, which would fire at 40 to 60 beats a minute, which is enough to retain consciousness in the resting state.

Types of SA nodal blocks include:

In addition to the above blocks, the SA node can be suppressed by any other arrhythmia that reaches it. This includes retrograde conduction from the ventricles, ectopic atrial beats, atrial fibrillation, and atrial flutter.

The difference between SA node block and SA node suppression is that in SA node block an electrical impulse is generated by the SA node that doesn't make the atria contract. In SA node suppression, on the other hand, the SA node doesn't generate an electrical impulse because it is reset by the electrical impulse that enters the SA node.

Types of AV nodal blocks

There are four basic types of AV nodal block:

Types of infrahisian block

Infrahisian block describes block of the distal conduction system. Types of infrahisian block include:

Of these types of infrahisian block, Mobitz II heart block is considered most important because of the possible progression to complete heart block.


Damage to the heart muscle and its electrical system by diseases, surgery, or medicines can cause acquired heart block. Heart block can be either congenital or acquired. Acquired heart block is more common than congenital heart block.

Risk Factors

The risk factors for congenital and acquired heart block are different.

Congenital Heart Block

  • If a pregnant woman has an autoimmune disease, such as lupus, her fetus is at risk for heart block.
  • Congenital heart defects also may result in congenital heart block. These defects are problems with the heart's structure that are present at birth. Most of the time, doctors don't know what causes these defects.
  • Heredity may play a role in certain heart defects. For example, a parent who has a congenital heart defect may be more likely than other people to have a child with the condition.

Acquired Heart Block

  • Acquired heart block can occur in people of any age. However, most types of the disorder are more common in older people. This is because many of the risk factors are more common in older people.
  • Exposure to toxic substances or taking certain medicines, such as digitalis, also can raise your risk of heart block.
  • Well-trained athletes and young people are at higher risk for first-degree heart block caused by an overly active vagus nerve. You have one vagus nerve on each side of your body. These nerves run from your brain stem all the way to your abdomen. Activity in the vagus nerve slows the heart rate.


Life Threatening Causes

Common Causes

Causes by Organ System

Cardiovascular No underlying causes
Chemical/Poisoning No underlying causes
Dental No underlying causes
Dermatologic No underlying causes
Drug Side Effect Aprotinin, , Fosphenytoin sodium, Interferon gamma, Methacholine, Pergolide
Ear Nose Throat No underlying causes
Endocrine No underlying causes
Environmental No underlying causes
Gastroenterologic No underlying causes
Genetic No underlying causes
Hematologic No underlying causes
Iatrogenic No underlying causes
Infectious Disease No underlying causes
Musculoskeletal/Orthopedic No underlying causes
Neurologic No underlying causes
Nutritional/Metabolic No underlying causes
Obstetric/Gynecologic No underlying causes
Oncologic No underlying causes
Ophthalmologic No underlying causes
Overdose/Toxicity No underlying causes
Psychiatric No underlying causes
Pulmonary No underlying causes
Renal/Electrolyte No underlying causes
Rheumatology/Immunology/Allergy No underlying causes
Sexual No underlying causes
Trauma No underlying causes
Urologic No underlying causes
Miscellaneous No underlying causes

Causes in Alphabetical Order



Symptoms depend on the level of the heart block:

Contraindicated medications

Heart block is considered an absolute contraindication to the use of the following medications:

See also


  1. Hurst's The Heart, 10th Edition. McGraw-Hill Professional Publishing. November, 2000. pp. Figure 24-60b. ISBN 0071356959. Unknown parameter |coauthors= ignored (help); Check date values in: |date= (help)

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